For example, JP-A-2001-16694 discloses a supersonic sensor device that is mounted on a vehicle such as car, and used for obstacle detection and the like. The supersonic sensor device has a transmission element and a reception element (one element may be used both as transmission and reception elements), and transmits a supersonic wave from the transmission element, and receives a supersonic wave that was struck to an obstacle and reflected thereby. The device is used for performing arithmetic processing and thus detecting a direction or distance of the obstacle, or determining unevenness of the obstacle using sound pressure, time difference, and phase difference of the supersonic wave received by the reception element.
As the reception element used for such a supersonic sensor device, an element has been noticed, in which an oscillator formed of a piezoelectric thin film is formed on a membrane formed as a small thickness portion of a substrate. The reception element having a membrane structure is formed by a semiconductor micromachining technique, and hereinafter called MEMS (Micro Electro Mechanical System) type supersonic sensor element. Such a MEMS type supersonic sensor element and a supersonic array sensor device using the element are disclosed, for example, in JP-A-2003-284182. The supersonic sensor element (reception element) is formed by sandwiching a thin film layer of a PZT ceramic that is a ferroelectric substance by two electrodes, and includes a piezoelectric sensor that has a predetermined resonance frequency to detect a supersonic wave, wherein predetermined bias voltage is applied between the two electrodes during measuring operation of the supersonic sensor element, thereby the resonance frequency of the element can be changed.
However, in the above configuration, a supersonic sound source and a supersonic array sensor device are separately provided, and manufacturing cost is necessary for each of them. Moreover, when they are equipped on a bumper of a car and the like, mounting accuracy of the supersonic sound source and the supersonic array sensor device affects on detection accuracy of the direction or distance of the obstacle, and an equipping interval between them becomes large.
In a supersonic sensor device directly mounted on the bumper of the car, when a water drop or dust is adhered on a surface of the supersonic sensor element, a distance to the obstacle cannot be accurately measured. Furthermore, attenuation of a supersonic wave propagated in air depends on temperature or humidity of air, and the temperature or humidity is different depending on ambient surrounding of the car, therefore there is a difficulty that detection accuracy of the obstacle is deteriorated by the effect of temperature change and humidity change. In particular, for temperature of the ambient surrounding of the car, ambient temperature can be measured by an ambient temperature sensor and the like, however for humidity, there is not any appropriate humidity sensor, which can be mounted outside a vehicle interior, consequently the difficulty is not eliminated.